Anisotropic magnets produced by milling crystalline magnetic anisotropy materials such as ferrites or rare-earth alloys and pressing the milled material in a specific magnetic field are widely used in speakers, motors, measuring instruments and other electrical devices. Of these, in particular, magnets with anisotropy in a radial direction are endowed with excellent magnetic properties, are freely magnetizable and require no reinforcement to fix the magnet in place as in the case of segment magnets, finding use in AC servomotors, DC brushless motors and other related applications. The trend in recent years toward higher motor performance has brought with it a demand for elongated radially anisotropic magnets. Magnets having a radial orientation are manufactured by vertical compacting in a vertical magnetic field or by backward extrusion. The vertical magnetic field, vertical compacting process is characterized by applying opposing magnetic fields through the core of a mold in the pressing direction so as to provide a radial orientation.
FIG. 1 illustrates a vertical magnetic field vertical compacting system for producing a radially anisotropic magnet. Illustrated in FIG. 1 are a compactor housing 1, coils 2 for generating orienting magnetic fields, a die 3, a top core 4, a bottom core 5, a top punch 6, a bottom punch 7, and a packed magnet powder 8. In this vertical magnetic field vertical compacting system (compactor), the magnetic fields generated by the coils create magnetic paths extending from the cores, through the die and the compactor housing and back to the cores. To reduce magnetic field leakage loss, a ferromagnet, typically a ferrous metal is used as the material making up the portions of the compactor that form the magnetic paths. However, the strength of the magnet powder-orienting magnetic field is determined as follows. Assume that B is a core diameter (magnet powder packed cavity inside diameter), A is a die diameter (magnet powder packed cavity outside diameter), and L is a magnet powder packed cavity height. Magnetic fluxes which have passed through the top and bottom cores meet from opposite directions at the core center, run against each other and divert into the die. The quantity of magnetic flux that passes through the core is determined by the saturation magnetic flux density of the core while an iron core has a magnetic flux density of about 20 kG. Therefore, the orienting magnetic fields at inside and outside diameters of a magnet powder packed cavity are obtained by dividing the quantity of magnetic flux which has passed through the top and bottom cores by the inside surface area and outside surface area of the magnet powder packed cavity, respectively. They are expressed by the following equations.2·π·(B/2)2·20/(π·B·L)=10·B/L (inner periphery)2·π·(B/2)2·20/(π·A·L)=10·B2/(A·L) (outer periphery)Because the magnetic field is smaller at the outer periphery than at the inner periphery, a magnetic field of at least 10 kOe is required at the outer periphery in order to obtain good orientation in all areas of the magnet powder packed cavity. As a result, 10·B2/(A·L)=10, and so L=B2/A. Given that the height of the powder compact is about one-half the height of the packed powder and is further reduced to about 80% during sintering, the magnet ultimately obtained has a very small height. Because the height of the magnet that can be oriented is dependent on the core shape, it is difficult to produce lengthy magnets by the method of producing a radial magnet in opposed magnetic fields using the vertical magnetic field vertical compacting system.
The backward extrusion process is not conducive to the production of low-cost magnets because of a large scale of equipment and low yields.
Thus, regardless of which process is used, radially anisotropic magnets are difficult to manufacture. The inability to achieve the low-cost, large-volume production of such magnets has in turn made motors that use radially anisotropic magnets very expensive to manufacture.
In order to manufacture a large number of elongated annular radial magnets in a multiple-cavity molding manner, the applicant proposed in JP-A 2004-111944 a method of manufacturing such radial magnets, without using a prior art vertical magnetic field vertical press, by applying a magnetic field in a horizontal magnetic field vertical press with a ferromagnetic core set in place, rotating the magnet powder relative to the magnetic field direction, applying a magnetic field again, and compacting, that is,
“a method of manufacturing radially anisotropic ring magnets in which a magnet powder packed into a cavity in a cylindrical magnet-forming mold having a core composed at least in part of a ferromagnetic material with a saturation magnetic flux density of at least 5 kG is pressed under the application of an orienting magnetic field by a horizontal magnetic field vertical compacting process; the method being characterized by carrying out at least one of the following operations (i) to (v):
(i) rotate the magnet powder a given angle in the circumferential direction of the mold during application of the magnetic field,
(ii) rotate the magnet powder a given angle in the circumferential direction of the mold following application of the magnetic field, then again apply a magnetic field,
(iii) rotate a magnetic field-generating coil a given angle in the circumferential direction of the mold with respect to the magnet powder during application of the magnetic field,
(iv) rotate a magnetic field-generating coil a given angle in the circumferential direction of the mold with respect to the magnet powder following application of the magnetic field, then again apply a magnetic field,
(v) use a plurality of coil pairs to first apply a magnetic field with one coil pair, then apply a magnetic field with the other coil pair
so as to apply to the magnet powder a magnetic field from a plurality of directions rather than one direction and pressure compaction, for thereby manufacturing a radially anisotropic ring magnet having throughout the magnet an angle of 80° to 100° between a center axis thereof and a radial anisotropy imparting direction.”
In this method, the magnetic field applied by placing a ferromagnetic core in a horizontal magnetic field press takes a radial orientation near a magnetic field applying direction as shown in FIG. 3b. At this point, it does not take a radial orientation in a direction perpendicular to the magnetic field applying direction. Then the packed magnet powder and the magnetic field applying direction are rotated relatively, after which a weaker magnetic field is applied to impart a radial orientation to those sites which have not taken radial orientation during the previous magnetic field application. Use of such a weaker magnetic field causes no disorders to the orientation in a direction perpendicular to the magnetic field applying direction. In this way, radial orientation is imparted throughout the circumferential direction. However, if the strength of the magnetic field applied immediately before compaction is too high, the radial orientation which has been established thus far is disordered in a direction perpendicular to the magnetic field. Also, if the strength is too low, the disordered orientation which has been induced during the latest application of a magnetic field in the magnetic field applying direction cannot be corrected into radial orientation. Therefore, whether or not uniform radial orientation is achieved largely depends on the strength of a magnetic field applied immediately before compaction. There thus exists a desire to have a more consistent production method.
Patent Reference 1: JP-A 2004-111944